Fender Pro Reverb Problem

[TimeBandit]

I'm trying to sort out why I'm not getting any reverb with a Fender Pro Reverb, circa 1971. The unit has been recapped (not by me), and the workmanship, component values, etc. look ok. Apparently, there was no reverb before the recap too. All of the tubes test ok, and the reverb transformer measures ok in terms of DC resistance: 800 ohms across the primaries, about 1 ohm across the secondaries. One thing that I'm suspicious of is the amplitude of a 1kHz signal before after the 500pF capacitor on the way to the reverb driver tube grid. My understanding was that combination of this cap and the 1 meg resistor will cut everything below 318Hz, and yet I'm seeing significant attenuation of a 1kHz as well as 2kHz signal. The resistor measures within 5 percent of spec, and I replaced the cap with no change.

The plate voltage of the 12AT7 driver is 434V, and cathode is 8.77V.

[Tom Phillips]

I'm trying to sort out why I'm not getting any reverb with a Fender Pro Reverb, circa 1971...]

Please try the following and let us know what the results are.
1. With the reverb front panel control turned up (#5 will be fine for the test) do you hear reverb crashing sound when you shake the reverb spring unit?
2. Reverse the leads of the RCA rear panel chassis connections. Does the reverb start working? Do you hear crashing when you shake the reverb spring unit?
3. Measure the DC resistance at the RCA plugs that connect the reverb tank to the chassis (With them unplugged) and tell us the readings you get.

These steps may be out of order based on the way others do the troubleshooting but the answers will tell us a lot about the status of your amp.

[TomCarlos]

Follow TP's test ideas.

And for good measure, please confirm the Reverb Transformer Part number (TR4) and the Reverb Pan type and part number.

[Jon Snell]

Don't over think what is happening.
Check you have signal on the cable to the pan.
Check the DC resistance of the pan send coil.
Check the reverb makes a noise when shaken/thumped, if not;
Check you have hum as you touch the nner wire of the return cable with your finger, adjusted with the reverb vol control.
Check the DC resistance of the recovery coil in the pan.
If not sorking but pan and cables check out OK, check for hum responce on the grid of the recovery valve, V4B. etc etc etc
There should be enough info gathered to pin point where the fault is.

[TimeBandit]

Please try the following and let us know what the results are.
1. With the reverb front panel control turned up (#5 will be fine for the test) do you hear reverb crashing sound when you shake the reverb spring unit?
2. Reverse the leads of the RCA rear panel chassis connections. Does the reverb start working? Do you hear crashing when you shake the reverb spring unit?
3. Measure the DC resistance at the RCA plugs that connect the reverb tank to the chassis (With them unplugged) and tell us the readings you get.

These steps may be out of order based on the way others do the troubleshooting but the answers will tell us a lot about the status of your amp.

Thanks Tom P. Results below:

1. With the Reverb control turned up to 5, I shook the reverb spring unit and heard a loud crashing sound.
2. Skipped switching the cables because of the results of #1.
3. Measured the resistance at approximately 0.3 ohms end to end, signal to signal, and ground to ground.

Tom C, I attached a pic of the TR4 part number (which was recently replaced by the owner of the amp). The spring unit is a Gibbs 64063 4U1.

[Tom Phillips]

See my comments in the quote section.

Thanks Tom P. Results below:

1. With the Reverb control turned up to 5, I shook the reverb spring unit and heard a loud crashing sound. That indicates that the reverb recovery circuit is working.
2. Skipped switching the cables because of the results of #1. I suggested that test as an easy way to determine if both tank transducers were working. The switch would only be temporary unless it resulted in the reverb working properly in which case it would indicate that the tank was hooked up backwards. (Note that either transducer will provide signal to the recovery circuit but only the input transducer will dive the springs enough to create reverb) The process is a quick way to do some diagnostics without using any test equipment.
3. Measured the resistance at approximately 0.3 ohms end to end, signal to signal, and ground to ground. That appears to indicate that you ohmed out the cables and that shows that the cables are OK. The goal was to measure the DC resistance of each transducer in the reverb tank. Since it appears that you have accessed the Gibbs tank, just measure the resistance across each of the tank's RCA connectors and look for broken wires.

Tom C, I attached a pic of the TR4 part number (which was recently replaced by the owner of the amp). The spring unit is a Gibbs 64063 4U1.

Looks lihe the tank is original.
The transformer is a Heyboer replacement for the Fender reverb drive transformer.
I suggest that you measure the cathode voltage of the 12AT7 reverb drive tube.

[TomCarlos]

If I am not mistaken (and according to my notes, taken from a different thread somewhere), for the Gibbs 64063 U1, the DC resistance at the input (right across the RCA connect on the pan) should be in the range of .8 to 2 ohms. Make sure you short your leads on the meter to account for internal meter and test cable resistance. The DC resistance at the output of the pan should be in the range of 165 to 270 ohms.

As Tom P mentions, recheck the voltage at the Cathodes of the 12AT7. It might be high (compared to the schematic) because the primary windings of the Heyboer are slightly different when compared to an original Fender transformer. So there could be a need to adjust the cathode resistor value to lower that voltage. This may NOT be the cause of your problem - but something to consider doing when using replacement parts (especially replacement transformers). I discovered that when trying to use a Fender Reverb transformer in a Peavey amp circuit.

[TimeBandit]

Ok, sorry about not swapping the cables in #2. It's a pet peeve of mine when people I'm trying to help don't follow the steps I lay out for them, and here I'm doing it.

I swapped the tank cables and heard crashing, though perhaps not as loud.

Measuring the DC resistance of each transducer, I get the following:

Input: 0.9 ohms
Output: 181 ohms

The voltage at the cathodes of the 12AT7 reverb driver is 8.81V.

Matt

[Tom Phillips]

Ok, sorry about not swapping the cables in #2. It's a pet peeve of mine when people I'm trying to help don't follow the steps I lay out for them, and here I'm doing it.
I swapped the tank cables and heard crashing, though perhaps not as loud.
Measuring the DC resistance of each transducer, I get the following:
Input: 0.9 ohms
Output: 181 ohms
The voltage at the cathodes of the 12AT7 reverb driver is 8.81V.
Matt

Good job Matt.
Now we know that the tank, the cables and the recovery circuit are OK.
The drive problem could be with the signal feeding the 12AT7 or the transformer circuitry. It's suspicious that the transformer had been replaced and that other work had been done on the amp but the reverb is still inoperative. I'd trace the signal driving the 12AT7. There could be a wiring or component problem. On your initial post you did mention some investigation you did in that part of the circuit. Let us know what you have for test equipment.

Touching the grid pin of the 12AT7 reverb driver with an open wire probe should produce a buzzy reverb sound if you feel absolutely safe about probing the live circuit like that.

[Helmholtz]

So there could be a need to adjust the cathode resistor value to lower that voltage.

No need to change the cathode resistor.

[Helmholtz]

Here's a good tip from late Enzo:
To check the drive circuit including the transformer connect a speaker to the reverb transformer secondary.
With a guitar (or other signal) connected to the reverb channel input you should get some sound as the reverb drive circuit is a little power amp.

[TomCarlos]

By the way.... you double checked the reverb On/Off footswitch, that it is working properly?

[Tom Phillips]

By the way.... you double checked the reverb On/Off footswitch, that it is working properly?

We can rule that out because the spring crashing sounds were getting through.

[Helmholtz]

While we're at it I have a question to the pro repair guys:
Do you always connect the tank or a dummy load to the rev. transformer when testing an amp with guitar or signal during a repair?
Background:
Every now and then we see a failed reverb transformer. It's not likely always a material or construction issue.
If loaded the only risk of transformer failure might be a shorted cathode cap or shorting tube causing excessive current. This would result in an open primary.
But without a load there could be a severe risk of excessive primary voltage leading to arcing and partially carbonized wire insulation (like with other tube power amps).
This might not necessarily cause immediate failure, but insulation strength will be lowered and next time exposed to high voltage stress shorted turns might develop.

[Tom Phillips]

While we're at it I have a question to the pro repair guys:
Do you always connect the tank or a dummy load to the rev. transformer when testing an amp with guitar or signal during a repair?...

I connect a spare 8 Ohm dummy load. For many years I left the reverb drive unconnected when I had the chassis on the bench. Then I had the same thought that you mentioned and started using the dummy load. The reverb drive transformer must be very robust in the common circuit application or we would be seeing very many more failures. Using the dummy load can only be better practice.